Semiconductor diode lasers are useful for a wide variety of applications, such as fiberoptic communications and optical data retrieval. Some potential applications, such as optical space communications, optical recording, semiconductor processing, spectrum analysis, laser radar, range finding and contour mapping, require high power, single spectral and spatial mode optical sources. Unfortunately, presently available diode laser configurations have difficulty in achieving both high power and a diffraction limited single mode beam in a single device. Diffraction limited single mode lasers are commercially available with an output power of 100 mW cw.
Phased array lasers are capable of achieving higher power. For example, a 10 W cw laser bar is commercially available. However, phased array lasers, in general, tend to have poor modal discrimination due to a variety of causes, including gain nonlinearities and spatial hole burning. These lasers operate in a diffraction limited single mode at low to moderate output powers, but as multimode oscillators at high power, some even at currents only slightly above threshold. Laser arrays that are most successful at maintaining a diffraction limited output do so only by suppressing other more favored modes, thereby limiting the achievable output.
The master oscillator power amplifier (MOPA) system is one structure that has been studied as having the potential to provide both high power and a diffraction limited single mode output. A MOPA with discrete oscillator and amplifier elements is described by Bradford et al. in U.S. Pat. No. 4,713,821. The MOPA includes a semiconductor laser diode having an output facet optically coupled to an optical power amplifier. The diode and amplifier are formed together as a single integral crystal, then cleaved to form a crack or division between the two. The diode and amplifier remain essentially a single unit in optical alignment with only a slight longitudinal shift resulting from the cleave. The active region at the entrance to the amplifier has the same lateral dimension as the active region at the exit of the laser diode, and the lateral dimension increases linearly in the forward direction toward the exit facet. The exit facet of the amplifier is antireflection coated.
In practice, MOPA structures with discrete elements are difficult to make, because the laser and amplifier must be mounted in precise alignment with close tolerances in order to achieve optical coupling. Further, even when alignment is achieved there are still substantial coupling losses to the amplifier because of the reflectivity of the entrance facet of the amplifier. Also, residual reflections of the amplifier excite self-oscillation in the amplifier, quenching the gain that is intended to be used by the laser output. As a result, a 100 mW cw diffraction limited output is the maximum limit.
In U.S. Pat. No. 4,744,089 to Montroll et al., an improved MOPA structure is described which requires no cleaving into separate elements, and is therefore monolithic. A laser diode has a periodic grating providing distributed feedback. A power amplifier with a diverging active area follows the laser, and the amplifier output facet is made antireflecting. The laser and amplifier are formed with separate electrical contacts so they can be driven by independent current sources. A power output of 1 W cw is achieved.
Feedback from the amplifier output facet must be minimized to allow the amplifier to be operated at high gain without parasitic oscillation deteriorating the spectral coherence of the device. In the MOPA structures described above, this is accomplished with an antireflection coating on the output facet. Unfortunately, a reflectivity below 0.001 is required and a reflectivity on the order of 0.0001 is preferred. Such low reflectivities are only achieved with difficulty with high quality dielectric coatings.
It is an object of the present invention to provide a monolithic master oscillator power amplifier (MOPA) structure which achieves both high power and diffraction limited, single spatial and spectral mode output beam.
It is another object to provide a MOPA structure in which the output beam is steerable.